Golden Freddy from Five Nights at Freddy's 2 is the scariest animatronic for various reasons:
-He only has one ear, which makes for a creepy and disturbing appearance.
-He is unpredictable, so the audience will automatically be startled, causing a scare factor.
-He is a hallucination, and therefore, he is able to manipulate your vision and possibly scare you to the point where you would get a heart attack or faint. Assuming he IS a hallucination, he is *scary* enough to cause heart attacks or the process of brief deoxygenation and passing out. If he isn't, then he would have to be a poltergeist. In this case, he is a reality warper that can change his appearance to a giant head at will and can cause death.
-He has no eyes in the 2nd game. This makes him look soulless and intimidating.

On the other hand, Springtrap is NOT scary because:
-He is a hopeless soul who cried out "help me" or "forgive me" in the hallways. And judging from the fact that there is no death screen, who even KNOWS if he kills you.
-He is obviously ready to jumpscare you when he looks at you through the hallway entrance.
-He isn't a hallucination and can't and can't warp reality, he is just a realistic human in a worn out suit.
-He has eyes, so he looks realistic and serious rather than creepy and intimidating.